The book compiles the story of the 12 development masters of LINE until they become line developers, and the process of growing up another level as developers with their colleagues in LINE. 12 developers, who are active in the world stage, joined LINE from various backgrounds, including teenage founders, game enthusiasts, philosophy majors and developers from Amazon, etc. It vividly captures the unique development life of 12 people, including those who are not easily found, such as open source managers and tech Evangelists, and also includes tips and study methods for those who want to become developers and grow into developers. An event of ‘Year-end Response’ to endure New Year’s greeting traffic in three different time zones in Japan, Taiwan, and Thailand on December 31st, a story of gathering 4 people from four countries in one car for live broadcasting, and a ‘LINE Expedition’ episode of being misunderstood as smugglers due to traveling with various phones for field testing, episodes of global IT companies are also interesting.